Many printing technologies require some sort of heated drum or roller. The most common example is the heated rollers used to create a hot nip in a laser printer fuser. A heated roller can also be used in an aqueous ink-jet printing system to decrease the drying time and thus increase printing speed. In the Xerox offset solid ink printing process, a heated drum is used to support the entire image prior to its transfer to the media. This elevated temperature maintains the ink in a viscoelastic state, which allows it to spread and penetrate into the media during transfer. This increases solid fill density, decreases pile height, and increases the durability of the prints.
These systems are unable to print until they have reached their respective operating temperatures. The time it takes to achieve the operating temperature is proportional to the temperature and the mass of the systemxe2x80x94both of which can be very high. By way of example, the temperature required in a solid-ink printing system is around 65-70 degrees centigrade, while that in a laser fusing system is in the range of 150-300 degrees centigrade. Because of these high temperatures and masses, a rather large power is typically required in order to keep warm-up time to an acceptable minimum. It is not uncommon to see heating systems with power requirements in the range of 300-800 watts and even much more. However, this high power does come at a cost. Higher wattage heaters result in larger and larger current draws on the AC line. AC lines all have a certain amount of impedance (or resistance to the current flow). This line impedance can cause the power to flicker when the heater turns on and if the heater is too large (for example, incandescent lights near the printer may flicker which is unacceptable to our customer). Therefore, the upper and lower power level is determined by the need for faster warm-up time without excessive flicker.
In order to meet the requirements of worldwide customers, the entire printer must be designed to operate at any line voltage that might be encountered. This can be done by designing multiple heaters with multiple power supplies. For instance, one heater/supply combination for US, one for Japan, and one for Europe. However, this reduces volume, increases cost, and introduces more complexity into the logistics of both manufacturing and service. Another method that is used is to design the printer with an auto-switching power supply that works between 87V (low line in Japan) and 265V (high line in Europe and Australia) and automatically detects what line voltage is being applied. This is advantageous because a single printer can be manufactured and distributed throughout the world. AC cycle-dropping is used to control the average power to the heaters. The design challenge in this case is to develop a heating system that can operate at 87-265VAC and meet the combined requirements of cost, warm-up time, and low flicker. Therefore a need exists for a drum heater able to achieve all of these design requirements.
A drum heater consisting of a plurality of vanes made preferably from mica material and having one or more separate heater wire channels controlled from an electrical cable is provided for heating the interior of a printer drum or fuser. The drum heater has one or more heater channels defined by separate heating element wires thereby creating different resistances. These are wound around the mica vanes. The various channels can be operated alone, in serial, in parallel, or in some combination thereof, which creates more possible heater fluxes than actual heater channels. In conjunction with AC cycle-dropping, these various heater fluxes can be used to optimize warm-up time while reducing flicker during both warm-up and steady-state operation as well as provide redundant heating for increased reliability and/or limp-mode operation until a service technician can fix a broken heater. The mica vanes are held together on both ends with mica end caps for assembly, electrical isolation and thermal isolation. The mica vanes additionally define protruding tabs for attachment of narrow mica panels which protect the element wire from handling damage and prevent a broken element wire from contacting the interior of the drum or fuser. Internal mica xe2x80x9cbafflesxe2x80x9d are further used to increase efficiency and/or vary the flux from center to ends of the drum or fuser.